The Bleed Air Supply System (BASS) on a gas turbine provides air from the compressor stages of the engine to the aircraft Environment Control System (ECS). The air is "bled" off from the compressor via bleed air ports or holes in the compressor housing. These bleed air ports are located over specific portions or stages of the compressor. Each compressor stage comprises a set of static vanes preceded by a set of rotating vanes which force air through the compressor. Each stage of the compressor is normally smaller than the preceding stage, thus, the pressure of the air increases as it passes from one stage to the next. However, the actual pressure of the air in any given stage is a function of many variables including, for example, the speed that the compressor core is rotating and the air pressure at the compressor intake.
Therefore, the pressure of air bled from one bleed port may change substantially as engine operating conditions change. Thus, if it is desirable to obtain bleed air having a relatively constant pressure, it is necessary to place bleed air ports at more than one location in the compressor and to use external valves to select between bleed ports.
The BASS system on an aircraft engine normally takes bleed air from two or three of the compressor stages and controls the pressure, temperature and flow of the air to the aircraft interface.
In FIG. 3, a known two port bleed air system is illustrated wherein controllable shutoff valve 304 controls the flow of bleed air from a first bleed port 310 in compressor 300 and check valve 324 controls the flow of bleed air from a second bleed port 320. Controllable valve 308 controls the flow of air to bleed air system output 318.